Warm winters bring challenges for walnuts and other crops

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By Craig W. Anderson

Farmers planning to plant new walnut orchards – and working in their current orchards – will probably need to consider ways to cope with an expected decline in winter chilling. Whatever considerations are required for walnuts may extend to other crops if the upcoming climate brings warmer winters to San Joaquin County.

Our story begins with walnuts, the county’s number four crop, valued at more than $290.3 million according to the county Agriculture Commissioner’s 2019 Crop Report.

At the recent virtual UCCE California Walnut Conference, Katherine Jarvis-Shean, a University of California orchard systems advisor, delved into the steps growers can take to address the oncoming threat of lower chilling hours. The “three takeaways” she mentioned included: Growers need to start “Getting comfortable with the chill-portions model, a relatively new way of counting chill accumulation in the winter”; “orchards will experience more ‘low chill’ winters in the future”; and “Chandler, and higher-chill varieties, will need help getting through warmer winters in the coming decades.”

Diminishing winter chill hinders walnut trees and trees of other nuts, including pistachios and almonds, from experiencing emerging dormancy in a satisfactory way; some buds can open earlier, resulting in nut-size irregularity.

And, said Jarvis-Shean, it’s well established that year over year warming winters is becoming a hot topic among the state’s walnut growers who are beginning to see the effects of warmer winters on their crop. “The trees are starting the race at different times in the spring,” she said. “They also finish in terms of maturity in the fall at different times as well, so you’ve got a wider maturity window.”

Jarvis-Shean said winters have been warming up, in particular over the past 40 years, when temperatures have increased by 1½ to 3 degrees. In both the Sacramento and San Joaquin valleys the temperature increase from the 1980s to 2060 is estimated to eventually average about 3.1 degrees based on information from 16 different global general circulation models.

That’s good news, “knowing the weather’s there and that significantly negative storms and winters both warm and cold, can happen. We just don’t know when they’ll happen,” said SJFB President David Strecker. “What’s causing climate change that will affect agriculture is a long-running battle of opinions without a final solution yet.”

But that doesn’t stop the search for warm winter solutions and the details that drive it.

Traditionally, chill hours have been tracked by a method that counts any hour of temperatures between 32 and 45 degrees. Now a more complicated “chill portions (dynamic model)” accounts for different temperatures with a different “chill value” which allows warm temperatures to be subtracted from chill accumulation.

The chill portions relate to dormancy maintenance being affected by hormones and transport capacity, along with oxidative stress and metabolism; hormones then interact with metabolism and oxidative stress and metabolism influence on oxidative stress. “All of these factors are influenced by winter chill,” Jarvis-Shean said, adding that warm winters delay bud break and decrease yield. And bud break is foreshadowed by a significant upsurge in the tree’s starch.

What does starch have to do with warmer winters? In warmer winters, trees adjust their starch-making system to keep starch low and sugars stable in mid-winter; however, then more heat is required in the spring to achieve high starch before bud break.

“It’s not just a matter of not getting enough cold,” she said. “It’s also a matter of not getting too much heat in the winter.”

Situations that affect bloom, starch and other aspects trees need to produce a crop can have long-lasting effects that produce a ripple effect across the crop, pointed out SJFB First Vice President Ken Vogel. “Certain numbers are needed for bloom in cherries. Walnuts require spraying be done at certain times and almonds are slightly closer to cherries as a bloom crop. So the effects of warm winters can be felt across multiple crops.”

Planting orchards now – almonds, walnuts, cherries – must take into consideration the possibility that warmer winters are expected to cause a 15% to 20% decline in chill portions in the Sacramento and San Joaquin Valleys by mid-century.

“Mid-century is one orchard life from now,” Jarvis-Shean said. “End of century is two orchard lives from now.” Time flies and, using Chandler walnuts as an example, they’ll have a 55 to 60 chill portions requirement and “those numbers could drop to between 42 and 51 by the end of the century. That’s not great news, but what can we do about it?”

Dormancy-breaking products are available and being tested by researchers with funding from the California Walnut Board; the products being tested include hydrogen cyanimide, a nitrogen cocktail and a hormone analogue.

“Cherry guys have been using these those materials for years,” said SJFB Second Vice President Jake Samuel. “We’ve been having warmer winters in cherries for the last ten years and dealing with dormancy and bud break in SoCal. These materials aren’t new, but they’re new to nuts.”

The materials allow for “a little more fine-tuning…of when we apply things, the rate at which we apply them, all that stuff,” said Jarvis-Shean. “But none of the three products being tested is labeled for use on walnuts in California, but they have a track record in some systems.” She thinks they could be labeled for use in three to four years.

Vogel said whenever the products are available and how often they’re used “could really affect the overhead of farming in negative ways. But who knows what’s ahead? However, it’s good to be prepared as much as possible.”

Other crops and cold

Mick Canevari, UCCE field crops advisor emeritus, said, “Climate change is incremental in its effects on varieties of grains such as wheat, barley, oats which can be bred for cooler conditions. Beans are a fairly warm weather crop that can be damaged by cold.”

Warm winters can definitely change our decision-making process, and Canevari added  “we’ve already gained valuable experience down south with warm winters. As cherries moved south farmers had to deal with climate adjustments. Cherry acreage is still growing in the southern part of the state with adjustments to cultural practices.”

“Years ago there were no dormancy breakers but ag is an elastic industry, farmers are crafty when changes need to be made as climate driven situations come along,” Canevari said. “We can’t control Mother Nature, but when hasn’t the climate changed?”

When crops move to a different area, they change due to the climate and, said Canevari, “Now we have the technology to change varieties to meet climactic changes. Agriculture as an industry is constantly evolving and changing as is the climate.”

“Farmers are a resilient group, able to adjust and adapt as needed to challenges, including climate change and warmer winters,” said SJFB Executive Director Blodgett said. “We’ll get through this as we always have.”